It is known that civil transport aircraft must be pressurized because, in cruise flight, an aircraft flies at an altitude which is often higher than 30,000 feet (approximately 9,000 meters), at which the external air has an oxygen content that is too low (and is also too cold and too dry) to be compatible with life. To pressurize the cabin of an aircraft, pressurization systems are installed in the aircraft to maintain a breathable atmosphere on board and in the cabin (which includes at least a cockpit and passenger areas within a fuselage of the aircraft). In particular, international aeronautical regulations demand that any public transport aircraft which flies at an altitude higher than 20,000 feet (approximately 6,000 meters) should be pressurized and that it establishes in the cabin an equivalent altitude which does not exceed 8,000 feet (approximately 2,400 meters) in normal flight.
It can however happen, following a succession of failures or an accident, that the pressurization of the aircraft can no longer be maintained at an acceptable level. An official procedure then obliges the pilot to make the aircraft descend, as quickly as possible, to a breathable altitude of 10,000 feet (approximately 3,000 meters). This procedure is called an emergency descent.
This situation demands a rapid reaction by the crew, particularly in the case of severe depressurization at high altitude, with an accelerated drop in the ambient oxygen content in the cabin.
European and American regulations impose performance criteria that all aircraft have to comply with for carrying out emergency descents. Emergency descent control systems are known which, as a primary function, assist the crew in the management of the emergency descent. These systems make it possible to manage the descent to restore an acceptable pressure. For this purpose, they generate rapid descents which are not optimized, notably with respect to other procedures that the aircraft must follow during this emergency descent phase.